The present invention relates to a metallic fiber board having catalytic functionality and to a process for making the same. Particularly, the invention relates to a process for the production of a catalyst-bearing metallic fiber board for catalyzing the combustion of hydrocarbons and suitable to be used in different hydrocarbon burning devices such as household boilers.
Household boilers produce heat through the combustion of hydrocarbon gases, though only methane has commonly been used in recent years. In traditional boilers, in order to achieve complete combustion of methane to prevent the products of incomplete combustion, such as CO, from being released into the atmosphere, temperatures of at least about 800.degree. C. are required. Such high temperatures, however, allow undesirable nitrogen oxide compounds to form, generally referred to as NO.sub.x, that cause serious pollution problems when emitted into the atmosphere. Various systems have been studied to lower the necessary temperature for complete combustion The lowest temperatures at which the complete combustion of methane has been obtained have required the use of catalysts, and these catalytic systems have successfully eliminated the formation of NO.sub.x compounds.
Generally, the catalysts in these systems are provided on a metallic or a ceramic substrate. Such substrates need to have very open structures in order to allow the air/methane mixture to pass easily. For instance, wire meshes, metallic or ceramic bodies provided with holes, and boards or meshes made of metallic or ceramic fibers have all been used. An article entitled "Performances of Premixed-Air Fibrous Burners with a Metallic and Ceramic Matrix" by Accornero et al., published in "CH.sub.4 Energia Metano", 1995, No.2, pages 20-33, discloses, for example, systems for lowering the methane combustion temperature involving the use of a fiberous catalyst formed into a board. The fibers in this example are produced from a Fe--Cr--Al alloy coated with an aluminum oxide (alumina) layer on which a catalytic layer of lanthanum manganite, LaMnO.sub.3, is depositted. The article, however, gives no account of how the LaMnO.sub.3 layer is prepared.
The patent application WO 97/02092 discloses another catalytic system obtained from a perforated foil or a metallic grid on which a porous ceramic layer, preferably of alumina or zirconia, is deposited by means of techniques including plasma spraying, flame spraying, and detonation spraying. The ceramic layer is then impregnated with a catalyst precursor solution or suspension. After suitable thermal treatments the final catalytic system is obtained. The production process described by this application, however, is disadvantageous in that only the side of the metallic substrate facing the spraying nozzles can be coated with the ceramic layer. Consequently, coating two opposite sides of a substantially flat body, such as a perforated foil, requires two successive coating phases. Further, the method is unworkable when applied to fibrous boards because the sprayed ceramic tends to adhere to the first contacted surface, namely the most external fibers, and does not effectively coat the interior fibers.
What is desired, therefore, is a board comprised of metallic fibers having catalytic functionality, as well as a process for producing such a board.